budget: Track ship, plane, land unit expenses in nat_budget[]

budget: Track ship, plane, land unit expenses in nat_budget[]

Extend struct budget member bm[] to cover ships, planes and land
units, too.

Plane maintenance changes because pilot pay is now consistently
rounded down.  Before it was rounded down for broke countries, else
up.  The stock game's pilots earn a little less than $25, and solvent
countries save $1 per plane.  The rounding doesn't make much sense
either way.  To be be addressed in a later commit.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>